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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis as a tool for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Scattering Probes of Material Structure

BYU physics researchers use some of the world's brightest x-ray and neutron sources to study the atomic structure and dynamics of advanced materials like superconductors and piezoelectrics. At the Advanced Photon Source at Argonne National Laboratory, electrons travel around a one-kilometer synchrotron ring at nearly the speed of light, emitting high-energy tangential Bremsstrahlung x-rays that are collimated into beams and scattered from material samples.

Generalized regular k-point grid generation on the fly

Wiley S. Morgan, John E. Christensen, Parker K. Hamilton, Jeremy J. Jorgensen, Branton J. Campbell, Gus L.W. Hart, and Rodney W. Forcade

In the DFT community, it is common practice to use regular k-point grids (Monkhorst-Pack, MP) for Brillioun zone integration. Recently Wisesa et al. (2016) and Morgan et al. (2018) demonstrated that generalized regular (GR) grids offer an advantage over traditional MP grids. The difference is simple but effective. At the same k-point density, GR grids have greater symmetry and 60% fewer irreducible k-points. GR grids have not been widely adopted because one must search through a large number of candidate grids; in many cases, a brute force search could take hours. This work describes an algorithm that can quickly search over GR grids for those that have the most uniform distribution of points and the best symmetry reduction. The grids are ∼60% more efficient, on average, than MP grids and can now be generated on the fly in seconds.

Normally supportive sublattices of crystallographic space groups

Miles A. Clemens, Branton J. Campbell, and Stephen P. Humphries

The tabulation of normal subgroups of 3D crystallographic space groups that are themselves 3D crystallographic space groups (csg's) is an ambitious goal, but would have a variety of applications. For convenience, such subgroups are referred to as `csg-normal' while normal subgroups of the crystallographic point group (cpg) of a crystallographic space group are referred to as `cpg-normal'. The point group of a csg-normal subgroup must be a cpg-normal subgroup. The present work takes a significant step towards that goal by tabulating the translational subgroups (a.k.a. sublattices) that are capable of supporting csg-normal subgroups. Two necessary conditions are identified on the relative sublattice basis that must be met in order for the sublattice to support csg-normal subgroups: one depends on the operations of the point group of the space group, while the other depends on the operations of the cpg-normal subgroup. Sublattices that meet these conditions are referred to as `normally supportive'. For each cpg-normal subgroup (excluding the identity subgroup 1) of each of the arithmetic crystal classes of 3D space groups, all of the normally supportive sublattices have been tabulated in symbolic form, such that most of the entries in the table contain one or more integer variables of infinite range; thus it could be more accurately described as a table of the infinite families of normally supportive sublattices. For a given pair of cpg-normal subgroup and normally supportive sublattice, csg-normal subgroups of the space groups of the parent arithmetic crystal class can be constructed via group extension, though in general such a pair does not guarantee the existence of a corresponding csg-normal subgroup.

High-pressure polymorphism in pyridine

Branton J. Campbell (et al.)

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z′ = 1 and phase III in P41212 with Z′ = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.

Topotactic, pressure-driven, diffusion-less phase transition of layered CsCoO₂ to a stuffed cristobalite-type configuration

Branton J. Campbell (et al.)

CsCoO₂, featuring a two-dimensional layered architecture of edge- and vertex-linked CoO₄ tetrahedra, is subjected to a temperature-driven reversible second-order phase transformation (α → β) at 100 K, which corresponds to a structural relaxation with concurrent tilting and breathing modes of edge-sharing CoO₄ tetrahedra. In the present investigation, it was found that pressure induces a phase transition, which encompasses a dramatic change in the connectivity of the tetrahedra. At 923 K and 2 GPa, β-CsCoO₂ undergoes a first-order phase transition to a new quenchable high-pressure polymorph, γ-CsCoO₂. It is built up of a three-dimensional cristobalite-type network of vertex-sharing CoO₄ tetrahedra. According to a Rietveld refinement of high-resolution powder diffraction data, the new high-pressure polymorph γ-CsCoO₂ crystallizes in the tetragonal space group I4₁/amd:2 (Z = 4) with the lattice constants a = 5.8711 (1) and c = 8.3214 (2) Å, corresponding to a shrinkage in volume by 5.7% compared with the ambient-temperature and atmospheric pressure β-CsCoO₂ polymorph. The pressure-induced transition (β → γ) is reversible; γ-CsCoO₂ stays metastable under ambient conditions, but transforms back to the β-CsCoO₂ structure upon heating to 573 K. The transformation pathway revealed is remarkable in that it is topotactic, as is demonstrated through a clean displacive transformation track between the two phases that employs the symmetry of their common subgroup Pb2₁a (alternative setting of space group No. 29 that matches the conventional β-phase cell).

Supercolossal Uniaxial Negative Thermal Expansion in Chloranilic Acid Pyrazine, CA-Pyz

Harold T. Stokes and Branton J. Campbell (et al.)

There has been significant recent interest in exploiting the large dimension changes that can occur in molecular materials as a function of temperature, stress, or under optical illumination. Here, we report the remarkable thermal expansion properties of chloranilic acid pyrazine co-crystals. We show that the compound shows uniaxial negative thermal expansion over a wide temperature range with a linear contraction coefficient as low as (−)1500 × 10^–6 K^–1 at 250 K. The corresponding 10% contraction between 200 and 300 K is an order of magnitude larger than in the so-called colossal contraction materials. We adopt a symmetry-inspired approach to describe both the structural changes that occur (using rotational symmetry modes) and the thermal expansion (using strain modes). This allows an extremely compact description of the phase transition responsible for this unusual behavior and gives detailed understanding of its atomic origins. We show how the coupling of primary and secondary strain modes in materials showing extreme expansion and contraction can lead to unusual reversals in the temperature dependence of cell parameters.